Liquid phase, i.e., wet, chemical etching is a typical technique for delineating patterns in blanket coatings. However, where higher resolution patterns are involved, gas phase chemical etching has become of greater interest. Gas phase chemical etching can be accelerated by forming a plasma of the reactant gas at low pressure. However, techniques such as the latter still involve etching by chemical reaction. In addition, if the by-products of the chemical reaction are not volatile under the low pressure conditions, a second etching step may be required to remove them. In some instances one may even need to repeat the first and second steps a multiplicity of times to etch completely through a coating.
Tin oxide is a difficult material to etch. It is resistant to attack by many liquid chemicals, requiring special procedures to etch it. For example, one can etch it with nascent hydrogen that is produced by dissolution of zinc in hydrochloric acid. In such a technique the nascent hydrogen reduces the tin oxide to free tin, which is then removed by reaction with additional hydrochloric acid that is not reacted with the zinc. Such an etching procedure does not produce good resolution and is difficult to control. Electrochemical etching, using a platinum counter-electrode and a hydrochloric acid electrolyte, is generally unsatisfactory in that it has a tendency to leave unetched patches. Sputter etching is not an acceptable alternative because of non-selectivity and low etch rates.
Tin oxide has previously been plasma etched using a hydrogen as the reactant gas. However, the reaction leaves a residue, probably free tin, in the etched areas that has to be etched off with an acid solution. Use of the acid solution, of course, limits the resolution obtainable. A chlorinated gas such as hydrogen chloride or boron trichloride could be substituted for the hydrogen. However, such gases are considered toxic and/or corrosive, and the etching still must be conducted in two steps, as with the hydrogen plasma etching.
On the other hand, I have found that tin oxide can be readily etched in one step by reactive ion etching if the reactant gas consists substantially of argon or nitrogen containing a small proportion of hydrogen. Moreover, this reactant gas is quite selective for tin oxide, in that it does not readily attack commercially available organic resists, as for example, Shipley's AZ 1350 J, Phillip Hunt Chemical Company's HPR204 and Kodak's 747. Further, the etchant gas does not form polymers when used that contaminate the vacuum system. Still further, the reactant gas does not interact with the masking resist in such a way as to make it more difficult to remove after etching is completed. After etching with my technique, resists can be readily removed by conventional techniques, as for example, removal by oxygen plasma or simply by rinsing with acetone. Accordingly, extremely high resolution, i.e., narrow width lines, can be etched. Since my technique does not produce significant undercutting, I refer to my technique as being highly anisotropic in character.
It should also be noted that the improvements of the technique of this invention are quite similar to those of the technique covered by my concurrently filed U.S. patent application Ser. No. 639,853, entitled "Reactive Ion Etching of Tin Oxide Films Using Neutral Reactant Gas Containing Hydrogen" and assigned to the assignee hereof.